Many prior art reports of ammoniacal leaching have used tank or vat leaching, in which the ore or concentrate is simply immersed directly into ammoniacal solution.
In the case of copper, these previous processes have been reported as failures (e.g. Arbiter process, Escondida, as discussed in Arbiter & McNulty, Copper '99, p. 197-212). Specifically, the failure of the Arbiter process is attributed to poor removal of sulphate ions from solution by a lime boil, the leach kinetics being too slow even at the elevated temperature of 75-100° C. due to lower operating oxygen pressure (138 kPa) than ideal (690 kPa) and a lower (2:1) weight ratio of ammonia to copper than ideal (3.5:1). The failure of the Escondida process is attributed to insufficient retention time due to slower than expected kinetics, steam recovery of ammonia being more difficult than expected, solid-liquid separation after leaching being ineffective, ammonia being extracted by the LIX54 solvent extraction reagent used. Largely as a result of these difficulties, the plant never reached design capacity.
For zinc, the Schnabel process was used for a number of years before being superseded by the more environmentally friendly acid-based roast-leach-electrowin process. The Schnabel process feed was typically roasted sphalerite flotation concentrate but selective mining also allowed processing of zinc oxide ores. The Schnabel process is complex (as is evident from the summary by Harvey (Mineral Processing & Extractive Metallurgy Review, volume: 27, pages: 231-279, 2006), and it is perhaps unsurprising that there are no Schnabel process plants in operation.
A similar process has also been used for the refining of nickel and cobalt. The major difference is that the nickeliferous feedstock has been prepared either by reductive roasting (Caron process) or by chemical precipitation from an acid solution (e.g. pressure acid leach plants such as Moa Bay, Murrin Murrin, etc). These processes share several common features                Elevated leaching temperature, typically in excess of 30° C.        Leaching solution concentrations of >50 g/L ammonia        Pregnant leach solution concentrations of >50 g/L metal        Use of steam to strip and recover both ammonia and carbon dioxide        
Further, the well known Sherritt-Gordon process for nickel matte dissolves >80% nickel matte in ammoniacal solutions but only after leaching at >120° C. under >5 atm of oxygen pressure, which is obviously highly energy intensive.
Although many methods for ammoniacal leaching are described in the literature, including the patent literature, all require one or more of multiple leaching stages, energy-intensive pre-treatment of the ore (for example roasting or grinding) and/or elevated leach temperatures and/or pressures. These requirements necessitate complicated engineering and significant energy consumption, so it is little surprise that none have been widely adopted.
Further, many ammoniacal leaching methods either suffer from significant ammonia loss, or require complicated chemistry or engineering to overcome such, providing a further impediment to the widespread adoption of ammoniacal leaching methods.
Ammoniacal leaching is particularly attractive for high carbonate ores, which are generally not amenable to acid leaching-based processes for economic reasons. However, the scope of the present invention is not limited to high carbonate ores.
The method of leaching of the present invention has as one object thereof to overcome the abovementioned problems associated with the prior art, or to at least provide a useful alternative thereto.
Throughout this specification, unless the context requires otherwise, the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
The discussion of the background art is included exclusively for the purpose of providing a context for the present invention. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was common general knowledge in the field relevant to the present invention in Australia or elsewhere before the priority date.